Transistor and method for forming the same

ABSTRACT

The present invention relates to a transistor and the method for forming the same. The transistor of the present invention comprises a semiconductor substrate; a gate dielectric layer formed on the semiconductor substrate; a gate formed on the gate dielectric layer; a channel region under the gate dielectric layer; and a source region and a drain region located in the semiconductor substrate and on respective sides of the channel region, wherein at least one of the source and drain regions comprises a set of dislocations that are adjacent to the channel region and arranged in the direction perpendicular to a top surface of the semiconductor substrate, and the set of dislocations comprises at least two dislocations.

This application claims priority under 35 U.S.C. §§120 and 365(c) as acontinuation application of prior International Application No.PCT/CN2011/000262, filed Feb. 21, 2011, and which was not published inEnglish under PCT Article 21(2). The PCT/CN2011/000262 application inturn claims priority to Chinese Application No. CN 201010532062.9, filedon Oct. 29, 2010. The disclosures of the prior international applicationand Chinese application are incorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to the field of semiconductor devicemanufacturing, and particularly, to a transistor and a method forforming the same.

BACKGROUND OF THE INVENTION

Typically, integrated circuits comprise a combination of N-type MetalOxide Semiconductor (NMOS) and P-type Metal Oxide Semiconductor (PMOS)transistors formed on a substrate. The performance of an integratedcircuit is directly related to the performance of transistors which itcomprises. Therefore, it is desirable to improve the driving current ofa transistor so as to enhance the performance thereof.

U.S. patent application No. 2010/0038685A1 discloses a transistor inwhich dislocations are formed between the channel region and thesource/drain region. The dislocations may cause tensile stress, whichmay improve the electron mobility in the channel and thus increase thedriving current in the transistor. FIGS. 12 a-12 c in the presentapplication show the formation of such dislocations. In FIG. 12 a,silicon is implanted into a semiconductor substrate 1 on which a gatedielectric layer 2 and a gate 3 have been formed, so as to formamorphous regions as indicated by the shaded portions in the figure.Then the semiconductor substrate 1 is annealed so as to re-crystallizethe amorphous regions. As indicated by the arrows in FIG. 12 b, thecrystal growth front in the horizontal direction and the crystal growthfront in the vertical direction meet during the process of there-crystallization. As a result, dislocations are formed as shown inFIG. 12 c.

SUMMARY OF THE INVENTION

An object of the present invention is to provide a transistor and amethod for forming the same.

The method for forming a transistor comprises

forming a gate dielectric layer on a semiconductor substrate;

forming a gate on the gate dielectric layer;

performing a first ion implantation with a first implantation depth in afirst region and a second region of the semiconductor substrate locatedin the substrate on respective sides of the gate;

performing annealing to form a dislocation in each of the first andsecond regions after the first ion implantation;

performing a second ion implantation with a second implantation depthsmaller than the first implantation depth on one or both of the firstand second regions; and

performing annealing after the second ion implantation to form adislocation in each of the first and second regions.

The method for forming a transistor according to the present inventionproduces more dislocations adjacent to the channel region as comparedwith the prior art by performing amorphorization and re-crystallizationwith different depths in the source and drain regions. More dislocationscreate more tensile stress applied to the channel, which makes itpossible to increase the electron mobility in the channel region.

The transistor according to the present invention comprises

a semiconductor substrate;

a gate dielectric layer formed on the semiconductor substrate;

a gate formed on the gate dielectric layer;

a channel region under the gate dielectric layer; and

a source region and a drain region located in the semiconductorsubstrate and on respective sides of the channel region,

wherein at least one of the source and drain regions comprises a set ofdislocations that are adjacent to the channel region and arranged in thedirection perpendicular to a top surface of the semiconductor substrate,and the set of dislocations comprises at least two dislocations.

Due to the fact that there are more dislocations adjacent to the channelregion in the transistor of the present invention as compared with theprior art, the tensile stress applied to the channel region is increasedand thus the electron mobility in the channel region can be furtherincreased.

Other aspects and advantages of the present invention will be detailedhereinafter in combination with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 schematically illustrates a transistor according to a firstembodiment of the present invention.

FIGS. 2 a-d schematically illustrate the steps of the method for formingthe transistor according to the first embodiment of the presentinvention.

FIG. 3 schematically illustrates a transistor according to a secondembodiment of the present invention.

FIG. 4 schematically illustrates one of the steps of the method forforming the transistor according to the second embodiment of the presentinvention.

FIG. 5 schematically illustrates a transistor according to a thirdembodiment of the present invention.

FIG. 6 schematically illustrates one of the steps of a method forforming a transistor according to a fourth embodiment of the presentinvention.

FIG. 7 schematically illustrates the transistor according to the fourthembodiment of the present invention.

FIG. 8 schematically illustrates one of the steps of the method forforming the transistor according to a variation of the fourth embodimentof the present invention.

FIG. 9 schematically illustrates the transistor according to thevariation of the fourth embodiment of the present invention.

FIG. 10 schematically illustrates one of the steps of the method forforming a transistor according to a further variation of the fourthembodiment of the present invention.

FIG. 11 schematically illustrates the transistor according to thefurther variation of the fourth embodiment of the present invention.

FIGS. 12 a-c illustrate the formation of dislocations in the prior art.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The preferred embodiments of the present invention will be detailed inthe following in combination with the accompanying drawings. Thedrawings are drawn schematically and not to scale for the purpose ofillustrating the embodiments of the present invention rather thanlimiting the protection scope thereof. The same reference numbers areused to indicate same or similar components throughout the drawings.Processes and device structures that are well known in the art areomitted here in order to make the technical solutions of the presentinvention clearer.

<The First Embodiment>

FIG. 1 schematically illustrates a transistor according to a firstembodiment of the present invention. As shown in FIG. 1, a transistor100 comprises a semiconductor substrate 102, a gate dielectric layer 104formed on the semiconductor substrate 102, a gate 106 formed on the gatedielectric layer 104, a source region 108 and a drain region 110 onrespective sides of the gate 106, and a channel region 112 under thegate dielectric layer 104 and between the source region 108 and thedrain region 110. In the transistor 100 shown in FIG. 1, each of thesource region 108 and the drain region 110 comprises a set ofdislocations that are adjacent to the channel region 112 and arranged inthe direction perpendicular to the top surface of the semiconductorsubstrate, and each set of dislocations comprises two dislocations 101.The dislocations apply tensile stress to the channel region 112 asindicated by the arrow in FIG. 1, which increases the electron mobilityin the channel region. Compared with the prior art, more dislocationsare formed adjacent to the channel region in the present invention, andthus the tensile stress applied to the channel region 112 is increased,which makes it possible to further increase the electron mobility in thechannel region 112.

In addition, the transistor 100 further comprises sidewall spacersformed on the side surfaces of the gate dielectric layer 104 and thegate 106, as well as source and drain contacts, etc., which are wellknown structures in the art and are thus neither shown in the drawingsnor described in detail.

The method for forming the transistor in accordance with the firstembodiment will be described hereinafter with reference to FIGS. 2 a-d.

As shown in FIG. 2 a, the gate dielectric layer 104 and the gate 106 areformed on the semiconductor substrate 102. Next, as shown in FIG. 2 b, afirst ion implantation with a first implantation depth d1 is performedin a first region 108 and a second region 110 that are located in thesemiconductor substrate 102 and on respective sides of the gate 106, sothat an amorphous region (as shown by the shaded portions in FIG. 2 b)is formed in both of the first region 108 and the second region 110. Thefirst region 108 and the second region 110 are regions in which thedrain and the source of the transistor are to be formed respectively, orregions in which the drain and the source of the transistor have beenformed by performing ion implantation. An annealing is performedsubsequent to the step shown in FIG. 2 b so that the amorphous regionsare re-crystallized. During the process of the re-crystallization,different growth fronts of the crystal meet, and as a result,dislocations (as shown in FIG. 2 c) are formed in both of the firstregion 108 and the second region 110 and adjacent to the region on whichthe conductive channel is to be formed under the gate dielectric layer104.

Next, a second ion implantation with a second implantation depth d2 thatis smaller than d1 is performed in the structure shown in FIG. 2 c, soas to form amorphous regions as shown in FIG. 2 d. The ion implantationdepth can be controlled by adjusting the energy and dose of the ionimplantation. Subsequently, an annealing is performed on the structureshown in FIG. 2 d, so as to obtain the structure shown in FIG. 1.

<The Second Embodiment>

FIG. 3 schematically shows the transistor according to the secondembodiment of the present invention. The transistor 200 shown in FIG. 3is different from the transistor 100 shown in FIG. 1 in that each of thesource region 108 and the drain region 110 comprises a set of threedislocations that are adjacent to the channel region 112 and arranged inthe direction perpendicular to the top surface of the semiconductorsubstrate 102.

Accordingly, compared with the method for forming the transistor 100,the method for forming the transistor 200 in accordance with the presentembodiment further comprises a step of performing a third ionimplantation in the first region 108 and the second region 110 with animplantation depth d3 that is smaller than the second implantation depthd2, as shown in FIG. 4.

Although FIG. 3 shows that each set of dislocations in the source region108 and the drain region 110 comprise three dislocations, the presentinvention is not limited thereto. Each set of dislocations in the sourceregion 108 and the drain region 110 may comprise more dislocations.Accordingly, said more dislocations can be formed by performing more ionimplantations with different implantation depths, and the implantationdepth of a later ion implantation is smaller than that of a former one.

According to the present embodiment, more dislocations can be formedadjacent to the channel region as required, and thus the tensile stressapplied to the channel region is further enhanced, which accordinglymakes it possible to further increase the electron mobility of thechannel region.

<The Third Embodiment>

Although the dislocations in the source region and those in the drainregion are symmetrical to each other in the above description of thefirst and second embodiments, the present invention is not limitedthereto. By completely covering one of the first and second regions witha mask layer before any one of the ion implantations and only performingthe ion implantation in the other thereof, dislocations can be formedasymmetrically in the source region 108 and the drain region 110. As anexample, before the second ion implantation, a mask layer is formed onthe first region 108 but not on the second region 110 so that the secondion implantation is not performed in the first region 108. In such away, the first region 108 comprises only one dislocation while thesecond region 110 comprises a set of two dislocations in the directionperpendicular to the top surface of the semiconductor substrate 102 inthe resulting transistor 200 a, as shown in FIG. 5.

<The Fourth Embodiment>

The method for forming the transistor according the present embodimentis different from those of the first and second embodiments in that amask layer may be selectively formed on at least one of the first region108 and the second region 110 before one or more of the ionimplantations, to cover a portion of the at least one of the firstregion 108 and the second region 110 and, in a preferred embodiment, aportion of the at least one of the first region 108 and the secondregion 110 that abuts on the gate is exposed. In a non-limiting example,in addition to carrying out the steps of the method in the firstembodiment, a mask layer 114 is formed on the second region 110 so as tocover a portion thereof before the second ion implantation. FIG. 6 showsa structure obtained from the second ion implantation subsequent to thestep of forming the mask layer 114, in which structure ion implantationis not performed to the portion of the second region 110 that is coveredby the mask layer 114. An annealing is performed on the structure shownin FIG. 6 to obtain the transistor 300 shown in FIG. 7. Although themask layer 114 is also shown in FIG. 7, it may have been removed beforethe annealing in practice.

The mask layer may be a photoresist layer or a hard mask layer formed ofa dielectric material such as silicon oxide and/or silicon nitride. Theselective formation of the hard mask layer may be performed by, forexample, the photolithography process that is well known in the art. Inthe case that mask layers are selectively formed before more than one ofthe ion implantations, the patterns of the mask layers formed at onetime may be the same or vary from one mask layer to another. In onepreferred embodiment, the mask layer is formed of a dielectric materialsuch as silicon oxide and/or silicon nitride, and thus there is no needto remove the mask layers during the process of annealing in the casewhere the patterns of the mask layers are the same, so that the step offorming the mask layer only need to be performed once.

As a variation of the fourth embodiment, a mask layer may be selectivelyformed on at least one of the first region 108 and the second region 110before one or more of the ion implantations so as to cover at least twoportions thereof that are not adjacent to each other. As a non-limitingexample, in addition to performing steps in the method of the firstembodiment, a mask layer 114 is formed on both of the first region 108and the second region 110 so as to cover two portions thereof that arenot adjacent to each other before performing the first and second ionimplantations, respectively. The first ion implantation is thenperformed and a structure is obtained as shown in FIG. 8. Next, thesecond ion implantation and a corresponding annealing are performedwithout removing the mask layer 114. It is to be noted that positions ofthe mask layer 114 on the first region 108 and the second region 110 maybe symmetrical or asymmetrical with respect to the gate 106 of thetransistor 100. The resulting transistor 100 a formed in this example isschematically shown in FIG. 9. Although the mask layer 114 is also shownin FIG. 9, it may have been removed before the annealing in practice.

In another non-limiting example, the mask layer 114 is formed only onone of the first region 108 and the second region 110 so as to cover twoportions thereof that are not adjacent to each other, while no masklayer is formed on the other one of the first region 108 and the secondregion 110, or the other one of the first region 108 and the secondregion 110 is completely covered by the mask layer.

In another variation of the fourth embodiment, a mask layer isselectively formed on one of the first region 108 and the second region110 to cover two portions thereof that are not adjacent to each otherbefore one or more of the ion implantations, while the mask layer isselectively formed on the other one of the first region 108 and thesecond region 110 to cover a portion thereof. In a non-limiting example,in addition to performing the steps of the method in the firstembodiment, before performing the second ion implantation, the masklayer 114 is formed on the first region 108 so as to cover a portionthereof, and the mask layer 114 is formed on the second region 110 so asto cover two portions thereof that are not adjacent to each other. Thesecond ion implantation is then performed so as to obtain the structureshown in FIG. 10. FIG. 11 schematically shows the resulting transistor100 b obtained in this example. Although the mask layer 114 is shown inFIG. 11, it may have been removed before annealing in practice.

In summary, the transistor according to the fourth embodiment comprisesat least a further dislocation in at least one of the source region andthe drain region that is farther from the channel region than thedislocations formed in the first and second embodiments.

The direction that is parallel to the surface of the substrate isdefined as the horizontal direction of the transistor, and the directionthat is perpendicular to the surface of the substrate is defined as thevertical direction of the transistor. Compared with the first, secondand third embodiments, the fourth embodiment and its variations furtherobtain more dislocations in the horizontal direction of the transistorin addition to more dislocations in the vertical direction of thetransistor, which makes it possible for the tensile stress applied tothe channel region (and thus the electron mobility of the channelregion) to be further increased.

The transistors in accordance with the first to fourth embodiments andthe variations thereof may be NMOS transistors.

In the method for forming the transistor in accordance with the first tofourth embodiments and the variations thereof, the semiconductorsubstrate may comprise an NMOS device region and a PMOS device region,and the methods for forming the transistors in accordance with thepresent invention are performed only in the NMOS device region.

In the first to fourth embodiments and the variations thereof, thetransistors may also comprise a semiconductor layer (not shown), forexample, a layer of Si, SiC, SiGe or Ge, on the source region 108 andthe drain region 110, which makes the dislocations not to be exposed toa free surface. Accordingly, the method for forming the transistorcomprises forming the semiconductor layer on the source and drainregions after the doping steps for forming the source and the drain. Thefact that the semiconductor layer makes the dislocations not to beexposed to a free surface prevents the decrease of the tensile stressdue to exposure of dislocations to a free surface.

In the first to fourth embodiments and the variations thereof, the ionsimplanted in the ion implantations may be one of Si, Ge, P, B and As orany combination thereof.

In the first to fourth embodiments and the variations thereof, theannealing temperature may be greater than 400° C., preferably 500°C.-900° C., and the annealing time may be in the range of severalseconds to several minutes.

The doping of the source and the drain, the formation of the sidewallspacer, the formation of the source/drain contact and so on that arewell known in the art may be performed after the steps of the methoddescribed in the first to fourth embodiments and the variation thereof,thereby forming a complete device.

Although in the above descriptions, the doping processes for forming thesource and the drain are performed after the formation of thedislocations, the present invention is not limited thereto. Thedislocations may be formed in any suitable stage, for example, thedislocations may be formed after the doping of the source and the drain.

Furthermore, the semiconductor substrate mentioned above may be asilicon substrate, a SiGe substrate, a SiC substrate or an III-Vsemiconductor (for example, GaAs, GaN, etc.) substrate. The gatedielectric layer may use one of SiO₂, HfO₂, HfSiO, HfSiON, HfTaO, HfSiO,HfZrO, Al₂O₃, La₂O₃, ZrO₂ and LaAlO or any combination thereof, and thematerial of the gate may be selected from a group comprising Poly-Si,Ti, Co, Ni, Al, W, the alloy thereof and the silicide thereof.

The transistor and the method for forming the same according to thepresent invention are described by way of exemplary embodimentshereinbefore. However, the scope of the present invention does notintend to be limited thereto. Any modifications or variations of theabove embodiments that can be contemplated by one skilled in the artwill fall within the scope of the present invention defined by theappended claims.

What is claimed is:
 1. A transistor, comprising a semiconductor substrate; a gate dielectric layer formed on the semiconductor substrate; a gate formed on the gate dielectric layer; a channel region under the gate dielectric layer; and a source region and a drain region located in the semiconductor substrate and on respective sides of the channel region, wherein at least one of the source and drain regions comprises a first set of dislocations that are adjacent to the channel region and arranged in the direction perpendicular to a top surface of the semiconductor substrate, and the set of dislocations comprises at least two dislocations, and wherein the bottom of the dislocation that is closer to the channel region is substantially flush with the top of the dislocation that is more far away from the channel region.
 2. The transistor according to claim 1, wherein at least one of the source and drain regions comprises at least one further dislocation that is farther from the channel region than the first set of dislocations.
 3. The transistor according to claim 1, wherein at least one of the source and drain regions comprises at least one further set of dislocations arranged in the direction perpendicular to the top surface of the substrate, and the at least one further set of dislocations comprises at least two dislocations farther from the channel region than the first set of dislocations.
 4. The transistor according to claim 1, wherein the dislocations apply tensile stress to the channel region between the source and drain regions, which increases the electron mobility in the channel region.
 5. The transistor according to claim 1, wherein the transistor is an NMOS transistor. 